It's not on the (best way to put it i can think of) Exception(for lack of better word) List

Just for reference the best word to describe it would probably be 'whitelist', the opposite of a blacklist.

And no, modified W3D files are not permitted by RenGuard as they can be used to cheat (ie: big head). Hence only models which have been added to the aforementioned whitelist after having being checked by the RenGuard team are permitted to be in your data folder.

You can either;
1) Play without RenGuard
2) Delete the modified pistol model
3) Beg the RenGuard team to add the model to the whitelist and wait to see what happens.

In reality 1 and 2 are your only viable options, at this point I doubt anything else is going to get added to the whitelist.